When you store your bag compressed it takes up very little space and you tend to forget all about it and what it represents. When you hang it uncompressed it's more volumnious and you see it more, it's more in your face. This reminds you of the good times you had on your last outing and motivates you to plan the next trip.

And there's the small detail of storing the insulating material uncompressed versus compressed which isn't good for it.